In certain electrostatic imaging systems, particularly where full color images are produced, transfer of a toned image from an intermediate member is a technical barrier. This is especially the case where the developer is a liquid toner, since then the surface of the intermediate member must be resistant to the chemical effects of the liquid of the toner.
In such color systems the intermediate image is formed on the intermediate member by successive transfer from the surface of a photoconductor member of images in plural colors, typically the primary subtractive colors of magenta, cyan, and yellow. These three images, and often a fourth image of black, are layered on top of each other on the intermediate member to achieve a full color image in the final size to be transferred to paper or other final substrate. This invention is directed to achieving substantially complete transfer of such image over a long operative life of the imaging apparatus. Material selection for the intermediate transfer member is critical to such performance. The material must have very specific electrical and surface properties which cannot change over time or with use. The two principle materials that function best are silicones and polyurethanes. Polyurethanes have excellent abrasion resistance for long life and easily tailored electrical properties so that they will function in electrostatic transfer from the photoconductive roller. However, they also have high surface energy which causes the toner to bond to the intermediate transfer member during second transfer to paper, which is achieved by heat and pressure and electrical bias toward the paper.
In contrast, silicones have very low surface energy which facilitates 100 percent transfer of toner to paper. There are several limitation of silicone. One is that it has very high inherent electrical resistivity that is not easily modified. Second is that it has low abrasion resistance which is further reduced when the silicones is exposed to the toner carrier fluid, such as a heavy mineral oil. Thus, the electrical resistivity of silicones requires that they can only be used in thin layers, but such thin layer wear away quickly.
Mixtures of silicone and other resins are previously known. U.S. Pat. No. 3,957,367 to Goel is illustrative. A polymer of urethane and silicone is not shown in such known prior art. U.S. Pat. No. 5,084,735 to Rimai et. al. teaches other release materials in an intermediate transfer member.